Project Summary: Hepatocellular carcinoma (HCC) is a major cause of mortality worldwide. Its etiology is complex, including infection by hepatitis B virus (HBV) and chronic dietary exposures to the mycotoxin aflatoxin B1 (AFB1). Genomic analyses of AFB1-associated human HCC reveal specific mutational signatures, e.g., Signature 24. Recently, John Essigmann, Lawrence Loeb, and co-workers used a mouse model of HCC tumorigenesis to show that a unique mutational spectrum appearing early after AFB1 exposure persists until tumors develop over a year later. It recapitulates important aspects of the human Signature 24 spectrum. In pre-neoplastic murine hepatocytes, 25% of the spectrum consists of G?T transversions in the sequence 5'-CGC-3', which also possesses epigenetic significance due to the potential for cytosine methylation. We seek to understand the chemistry and biology driving mutation signatures associated with exposures to AFB1. AFB1 adducts in DNA rearrange to AFB1-formamidopyrimidine (AFB1-FAPY) adducts, which are genotoxic, yielding primarily G to T transversions. AFB1-FAPY exists as a mixture of configurational and conformational isomers. We recently showed that the human Base Excision Repair (BER) glycosylase NEIL1 plays a key role in removing AFB1- FAPY damage from DNA?humans deficient in NEIL1 may be at risk for AFB1-induced HCCs. This competing continuation application is premised upon the need to evaluate how NEIL1-initiated BER and error-prone bypass of AFB1-FAPY adducts is modulated by adduct isomeric equilibria and cytosine methylation status. The fundamental hypothesis is that responses to AFB1-FAPY damage depend upon sequence-specific differences in this mixture. If the aims of this application are successful, we will obtain sequence-specific structure-activity relationships for AFB1-FAPY isomeric adducts, with respect to DNA base excision repair (BER) and error- prone replication, which may act as drivers for unique AFB1-based mutational signatures observed in HCC. Additionally, our work may identify new targets for chemotherapeutic intervention in HCC. !